As the waveguide, either a single waveguide or two or more parallel waveguides can be used, which extend downward into the container from a point above the highest level to be measured. Bare metal wires, also known as Sommerfeld waveguides, are suitable for instance as the waveguides, or else metal wires provided with an insulation. The latter are also known as Goubau waveguides.
An electronic circuit for generating electromagnetic signals, and a reception and evaluation circuit for determining a fill level, are described for instance in European Patent Disclosure EP-A 780 665.
Level meters that employ electromagnetic signals can be used in many applications, both in the field of storage and in the processing industry, such as in chemistry, the foods industry, and the oil industry.
The level meter is exposed to often great temperature fluctuations, and strong tension or compression forces can act on the waveguide. There are many applications in which despite these adverse conditions, the product absolutely must be prevented from being able to escape through the meter.
In European Patent Disclosure EP-A 928 955, a level meter for measuring a level of a product in a container by means of electromagnetic signals is described which includes:                at least one waveguide protruding into the container,                    which carries the signals into the container and carries signals reflected from a product surface back out, and            which is secured in a housing that can be mounted on the container.                        
In the housing, inserts of a dielectric, such as a thermoplastic, a pressure-setting plastic, an elastomer, a ceramic, polyetherimide (PEI), polytetrafluoroethylene (PTFE), polyphenyl sulfide (PPS), or polycarbonates are provided, by which the waveguide is secured in the container. The housing, waveguide and inserts have conical jacket faces, which are disposed relative to one another such that they prevent a motion of the inserts and of the waveguide into the container. On a side remote from the container, the housing is closed off by a metal insert, by which a motion of the waveguide and inserts in the direction remote from the container is prevented. Spring elements are provided, by which the metal insert is pressed in the direction toward the container, and the inserts with the waveguide wedged into them are pressed in the direction away from the container.
For sealing purposes, in one exemplary embodiment, O-rings are provided, which are disposed in grooves between the housing and one of the inserts and between that insert and the waveguide.
Because of the fastening by means of springs, the insert, waveguide and housing can move slightly relative to one another. This can adversely affect the sealing action.
The waveguide, insert and housing form a coaxial cable in which the electromagnetic signals are carried. The grooves and O-ring represent discontinuities at which the impedance of the coaxial cable changes abruptly. Such abrupt changes in impedance cause reflection of a portion of the electromagnetic signals. That portion is subsequently no longer available for level measurement and instead forms interference signals, resulting in a markedly worse signal-to-noise ratio.
In another exemplary embodiment, in the side toward the container, a conical face is provided both between the waveguide and one insert and between that insert and the housing, and with this face the waveguide rests closely against an insert and that insert rests closely against the housing. Once again, a sealing action is attained thereby.
It is a problem in general that all the load-bearing parts must comprise rigid materials, if they are to perform their supporting function. If the level meter is exposed to major temperature changes, then these parts will shrink or expand in accordance with their coefficients of thermal expansion. While an expansion upon heating can be well compensated for by the spring construction, sealing problems can occur at low temperatures. Because of the temperature-dictated smaller size of the components, the spring prestressing is less, and tension and compression forces can cause gaps to form between sealing faces that in the normal state rest closely against one another.